Metal chips, such as aluminum chips that are the by-product of machining aluminum castings used in the automotive industry, have the unique benefit of being able to be re-melted back into the same metal as they started. For example, casted aluminum automobile wheels are machined into final products requiring the use of machining coolant, and as a result of this process, a significant percentage of the casted aluminum is converted to scrap chips. A manufacturing company that produces thousands of aluminum wheels thus creates thousands of pounds of scrap metal that can be reused if properly melted.
Efficient re-melting processes require properly sized chips that are essentially free of moisture. Drying the chips is important because moisture can interfere with magnetic separation processes that are required to remove undesirable levels of contaminant metal such as iron. And, chips that are essentially free of moisture must be fed to the molten bath for at least safety reasons.
While dry unoxidized chips are desirable, drying the chips to remove moisture can be an expensive process. For example, in many commercial processes, aluminum chips are dried in a direct-fired rotary drum to remove the moisture prior to being introduced into the feed system of melt furnace. Direct-fired rotary drum drying, however, requires large quantities of heat energy.
And, it has been observed that some of the chips may be destroyed, prior to melting, by converting a significant percentage of the chips to aluminum oxides that must be removed from the furnace as waste. In other words, it is known that aluminum chips resulting from direct-fired processes produce lower yields of usable molten metal due to the formation of high percentages of aluminum oxides. Thus, some drying processes can cause an economic loss. Also, it has also been observed that direct-fired drying substantially removes residual oils from the chips prior to delivery to the feed system of the furnace.
One known system and method for more efficiently drying metal chips prior to melting them is disclosed in U.S. Pat. No. 4,721,457, which issued on Jan. 26, 1988 and which is incorporated herein by reference. This system employs the use of a centrifuge that uses centrifugal force to remove 80 to 90% of the moisture and oil from the chips before indirectly heating the chips to a level high enough to vaporize the remaining moisture while leaving an appreciable amount of the oil on the chips until they are charged into the feed system of a furnace where the oil is at least partially combusted. It is believed that the presence of these oils creates a reducing atmosphere within the furnace feed system, thereby reducing the formation of aluminum oxides.